1. Field of the Invention
The present invention relates to an electric power steering control apparatus mounted on vehicles. In particular, the present invention relates to a field weakening control technique for improving an output from a motor.
2. Description of the Related Art
A typical electric power steering apparatus provides an assisting force (steering assisting force) by means of an electric motor when a driver steers a steering wheel of a vehicle. FIG. 11 shows a diagram illustrating a system configuration of an example of the electric power steering apparatus. In this figure, a reference numeral 14 indicates an electric motor that generates an assisting force, and such as a brushless motor is used as this electric motor. A reference numeral 13 indicates a current sensor that senses current flowing through the motor 14, and a reference numeral 15 indicates an angle sensor that senses a rotational angle of the motor 14. A reference numeral 20 indicates a steering wheel provided in front of a driving seat in a vehicle, a reference numeral 21 indicates a shaft one end of which is coupled to the steering wheel 20, a reference numeral 22 indicates a torque sensor that senses a torque applied to the shaft 21 when steering, and a reference numeral 23 indicates a gearing mechanism which is coupled to the other end of the shaft 21. A reference numeral 25 indicates a transmission mechanism that transmits a steering force of the steering wheel 20 and an assisting force of the motor 14 to a wheel 26 via a gearing mechanism 23. A reference numeral 28 indicates a vehicle speed sensor that senses a drive speed of the vehicle. A reference numeral 29 indicates a battery that supplies power voltage for driving the motor 14, a reference numeral 30 indicates a booster circuit that boots the voltage of the battery 29, and a reference numeral 31 indicates a controller constituted by an electronic control unit (ECU) that controls the motor 14.
Upon steering of the steering wheel 20, a torque generated at the shaft 21 due to the steering is detected by the torque sensor 22. A command value for current to be fed to the motor 14 is calculated by the controller 31 based on a value for the torque and a value for vehicle speed detected by the vehicle speed sensor 28. Then, current is supplied from a motor driving circuit incorporated in the controller 31 to the motor 14 based on the command value for current, and the motor 14 is rotated to generate the assisting force for steering the steering wheel 20. The current that flows through the motor 14 is detected by a current sensor 13, and a rotational angle of the motor 14 is detected by an angle sensor 15. These detected values are inputted to the controller 31. The controller 31 performs a feedback control so that a value for the current that flows through the motor 14 is equal to the command value.
FIG. 12 shows a graph indicating general characteristics of the motor in the electric power steering apparatus. As can be seen from the drawing, a rotating speed and a torque of the motor are conflicting, and the torque decreases as the rotating speed increases, and the torque increases as the rotating speed decreases. When the torque decreases, the steering wheel cannot be handled smoothly because the assisting force is not sufficient. When the rotating speed decreases, steering response deteriorates. Either case leads to unfavorable steering feeling.
Output characteristics required for a motor vary depending on a driving state of a vehicle. For example, because friction between a wheel and a road surface is small when the vehicle drives normally, the assisting force required for the steering is not very large. On the other hand, the friction between the wheel and the road surface is large when the vehicle is parked, and accordingly, the assisting force required for the steering is greater.
With a motor having properties as shown in FIG. 12, when steering while the vehicle is parked, only a torque Tb can be obtained where the torque required for assisting the steering for a motor rotating speed Nb is Ta. Thus, an output from the motor is insufficient and the assisting force required for the steering cannot be obtained, and handling of the steering wheel is not smooth. The same can be applied when the vehicle drives very slowly. On the other hand, when steering while the vehicle drives normally, only a rotating speed Nb can be obtained where a rotating speed necessary to ensure steering response for the torque Tb is Na. Thus, an output from the motor is also insufficient and a favorable steering response cannot be obtained.
An application of field weakening control has been proposed for weakening a magnetic field of the motor in a small region of the torque (see Japanese Patent Application Laid-Open No. 2002-145096, Japanese Patent Application Laid-Open No. 2003-40128, Japanese Patent Application Laid-Open No. 2004-40883, and Japanese Patent Application Laid-Open No. 2005-145342, for example). FIG. 13 shows an example of the characteristics of the motor when a field weakening control is applied. In the field weakening control, controlling field current weakens a magnetic field flux of the motor to decrease a back electromotive force, thereby improving an output of the motor. As a result shown in FIG. 13, the torque for the rotating speed Nb in the region where the field weakening control is performed is greater than a case without the field weakening control (Tb→Ta), and the rotating speed for the torque Tb in the region where the field weakening control is performed is greater than a case without the field weakening control (Nb→Na). Consequently, it is possible to obtain a required torque Ta to secure a sufficient assisting force when steering while the vehicle is parked, and it is possible to obtain a required rotating speed Na to provide a sufficient steering response when steering while the vehicle is driving.
However, when performing the field weakening control, the region in which the magnetic field is weakened fails to demonstrate linear characteristics as shown in FIG. 13 in practice, and demonstrates nonlinear characteristics as shown in FIG. 14. When the field weakening control makes the characteristics of the motor nonlinear as described above, it is not possible to realize a linear control and to obtain an output from the motor according to the command value. As a result, the steering feeling when steering while driving deteriorates.